1. Field of the Invention
The present invention relates to a load control device for controlling the amount of power delivered to an electrical load, such as a lighting load, and more particularly, a phase-control-to-DC converter circuit for determining a target intensity level for the lighting load from a phase-control signal.
2. Description of the Related Art
In order to save energy, high-efficiency lighting loads, such as, for example, compact fluorescent lamps (CFLs) and light-emitting diode (LED) light sources, are being used in place of or as replacements for conventional incandescent or halogen lamps. High-efficiency light sources typically consume less power and provide longer operational lives as compared to incandescent and halogen lamps. FIG. 1 is a simplified block diagram of a prior art lighting control system 10 having a screw-in compact fluorescent lamp 20. The screw-in compact fluorescent lamp 20 comprises a fluorescent lamp tube 22, which may be formed in a spiral (as shown in FIG. 1). The screw-in compact fluorescent lamp 20 also comprises an enclosure 24 for housing a load regulation circuit 40 (FIG. 2), e.g., an electronic ballast circuit, which is electrically coupled to the lamp tube 22 for illuminating the lamp tube. The screw-in compact fluorescent lamp 20 has a screw-in base 26 adapted to be coupled to a standard Edison socket. The lamp tube 22 of a typical prior art screw-in compact fluorescent lamp 20 is filled with a fill gas of 100% argon at a pressure of approximately 4 Torr.
The lighting control system 10 also comprises a “two-wire” dimmer switch 30, which is coupled in series between an alternating-current (AC) power source 15 and the screw-in compact fluorescent lamp 20 for controlling the intensity of the lamp tube 22. The dimmer switch 30 may be adapted to be mounted to a standard electrical wallbox and comprises a hot terminal H coupled to the AC power source 15 for receiving an AC mains line voltage VAC, and a dimmed-hot terminal DH coupled to the screw-in compact fluorescent lamp 20. The dimmer switch 30 does not require a direct connection to the neutral side N of the AC power source 15. Examples of prior art dimmer switches are described in greater detail is commonly-assigned U.S. Pat. No. 5,248,919, issued Sep. 29, 1993, entitled LIGHTING CONTROL DEVICE; U.S. Pat. No. 6,969,959, issued Nov. 29, 2005, entitled ELECTRONIC CONTROL SYSTEMS AND METHODS; and U.S. Pat. No. 7,687,940, issued Mar. 30, 2010, entitled DIMMER SWITCH FOR USE WITH LIGHTING CIRCUITS HAVING THREE-WAY SWITCHES, the entire disclosures of which are hereby incorporated by reference.
The dimmer switch 30 comprises a bidirectional semiconductor switch 32 coupled between the hot terminal H and the dimmed hot terminal DH for conducting a load current ILOAD through the screw-in compact fluorescent lamp 20. The bidirectional semiconductor switch 32 may comprise a single device, such as a triac, or a combination of devices, such as, two field-effect transistors (FETs) or insulated-gate bipolar junction transistors (IGBTs) coupled in anti-series connection. To control the amount of power delivered to the lamp tube 22, the bidirectional semiconductor switch 32 is controlled to be conductive and non-conductive for portions of a half-cycle of the AC power source 15, such that the bidirectional semiconductor switch is rendered conductive for a conduction time TON each half-cycle. The dimmer switch 30 may comprise a toggle actuator for turning the high-efficiency lamp tube 22 on and off and an intensity adjustment actuator for adjusting the intensity of the lamp tube 22 between a low-end intensity and a high-end intensity.
The dimmer switch 30 further comprises a control circuit 34 coupled in parallel with the bidirectional semiconductor switch 32 for conducting a control circuit ICNTL through the screw-in compact fluorescent lamp 20. The control circuit 34 is coupled to a control input of the bidirectional semiconductor switch 32 for rendering the bidirectional semiconductor switch conductive and non-conductive to generate a phase-control voltage VPC using either the forward phase-control dimming technique or the reverse phase-control technique. Accordingly, the bidirectional semiconductor switch 32 is rendered conductive for the conduction time TCON each half-cycle, thus setting a duty cycle DCPC of the phase-control voltage VPC. The control circuit 34 may be operable to provide, for example, a constant gate drive to the bidirectional semiconductor switch 32, such that the bidirectional semiconductor switch will remain conductive independent of the magnitude of the load current ILOAD conducted through the bidirectional semiconductor switch and the screw-in compact fluorescent lamp 20. An example of a two-wire dimmer switch having a constant gate drive control circuit is described in greater detail in commonly-assigned, co-pending U.S. patent application Ser. No. 12/952,920, filed Nov. 23, 2010, entitled TWO-WIRE DIMMER SWITCH FOR LOW-POWER LOADS, the entire disclosure of which is hereby incorporated by reference.
The screw-in base 26 of the compact fluorescent lamp 20 provides for only two electrical connections: a phase-control connection PC to the dimmer switch 30 for receiving the phase-control voltage VPC and a neutral connection NC to the neutral side N of the AC power source 15. The load regulation circuit 40 of the screw-in compact fluorescent lamp 20 is operable to adjust the intensity of the lamp tube 22 between the low-end intensity and the high-end intensity in response to the duty-cycle DCPC of the phase-control signal VPC (i.e., the conduction time of the bidirectional semiconductor switch 32 of the dimmer switch 30).
With forward phase-control dimming, the bidirectional semiconductor switch 32 is rendered conductive at some point within each AC line voltage half-cycle and remains conductive until approximately the next voltage zero-crossing, such that the bidirectional semiconductor switch is conductive for the conduction time each half-cycle. A zero-crossing is defined as the time at which the AC line voltage transitions from positive to negative polarity, or from negative to positive polarity, at the beginning of each half-cycle. Forward phase-control dimming is often used to control energy delivered to a resistive or inductive load, which may include, for example, an incandescent lamp or a magnetic low-voltage transformer. The bidirectional semiconductor switch of a forward phase-control dimmer switch is typically implemented as a thyristor, such as a triac or two silicon-controlled rectifiers (SCRs) coupled in anti-parallel connection, since a thyristor becomes non-conductive when the magnitude of the current conducted through the thyristor decreases to approximately zero amps. Thyristors are typically characterized by a rated latching current and a rated holding current, and comprise two main terminals and a control terminal. The current conducted through the main terminals of the thyristor must exceed the latching current for the thyristor to become fully conductive. In addition, the magnitude of the load current ILOAD conducted through the main terminals of the thyristor must remain above the holding current for the thyristor to remain in full conduction.
The control circuits of many forward phase-control dimmers comprise analog control circuits (such as timing circuits) for controlling when the thyristor is rendered conductive each half-cycle of the AC power source. The analog control circuit typically comprises a potentiometer, which may be adjusted in response to a user input provided from, for example, a linear slider control or a rotary knob in order to control the amount of power delivered to the lighting load. The analog control circuit is typically coupled in parallel with the thyristor and conducts a small timing current through the lighting load when the thyristor is non-conductive.
With reverse phase-control dimming, the bidirectional semiconductor switch 32 is rendered conductive at the zero-crossing of the AC line voltage and rendered non-conductive at some point within each half-cycle of the AC line voltage, such that the bidirectional semiconductor switch is conductive for a conduction time each half-cycle. The bidirectional semiconductor switch of reverse phase-control dimmers typically comprises two field-effect transistors (FETs) in anti-serial connection, or the like. Accordingly, prior art reverse phase-control dimmer switches have required advanced control circuits (such as microprocessors) for controlling the operation of the FETs, and power supplies for powering the microprocessors. In order to properly charge, the power supply of such a two-wire dimmer switch must develop an amount of voltage across the power supply and must conduct a charging current from the AC power source through the electrical load, in many instances even when the lighting load is off.
FIG. 2 is a simplified block diagram of the load regulation circuit 40 of the prior art screw-in compact fluorescent lamp 20. The load regulation circuit 40 comprises an electromagnetic interference (EMI) filter 50 for preventing noise generated by the load regulation circuit from being conducted on the AC mains wiring. A full-wave bridge rectifier 52 receives the phase-control voltage VPC from the EMI filter 50 and generates a rectified voltage VRECT. The rectified voltage VRECT is coupled to a bus capacitor CBUS through a diode D54 for generating a direct-current (DC) bus voltage VBUS across the bus capacitor. The load regulation circuit 40 further comprises an inverter circuit 56 for generating a high-frequency square-wave voltage VSQ from the rectified voltage VRECT, and a resonant tank circuit 58 for receiving the square-wave voltage VSQ and producing a substantially sinusoidal high-frequency AC voltage VLAMP (i.e., an arc voltage or lamp voltage), which is provided to the lamp tube 22. The inverter circuit 56 adjusts the operating frequency fOP of the square-wave voltage VSQ in order to adjust the intensity of the lamp tube 22.
The load regulation circuit 40 further comprises a phase-to-DC converter circuit 60 for receiving the rectified voltage VRECT and generating a DC voltage VDC that has a magnitude that is representative of the duty-cycle DCPC of the phase-control signal VPC, and a lamp current sense circuit 62 that generates a lamp current control signal VILAMP representative of a magnitude of a lamp current ILAMP conducted through the lamp tube 22. A control circuit 64 is coupled to the inverter circuit 56 for adjusting an operating frequency fOP of the square wave voltage VSQ and thus the magnitude of the lamp current ILAMP in response to the duty-cycle DCPC of the phase-control signal VPC and the magnitude of the lamp current ILAMP. The load regulation circuit 40 also comprises a power supply 66 that receives the bus voltage VBUS and generates a DC voltage VCC for powering the control circuit 64.
Since the dimmer switch 30 is a two-wire dimmer switch, the compact fluorescent lamp 20 receives both power for energizing the lamp tube 22 and information for determining the target intensity of the lamp tube from the phase-control signal VPC. The phase-to-DC circuit 60 typically comprises a filter circuit for preventing voltage fluctuations in the AC mains line voltage VAC of the AC power source 15 or noise on the AC mains line voltage VAC from altering the magnitude of the DC voltage VDC generated by the phase-to-DC converter circuit 60. Therefore, there is typically a delay time period between a change in the duty-cycle DCPC of the phase-control signal VPC and a resulting change in the magnitude of the DC voltage VDC generated by the phase-to-DC converter circuit 60. If the intensity adjustment actuator of the dimmer switch 30 is controlled such that the target intensity is quickly reduced from the high-end intensity to the low-end intensity, the magnitude of the phase-control signal VPC (and thus the amount of power available to the load regulation circuit 40) will quickly decrease while the control circuit 64 is still controlling the intensity of the lamp tube 22 to the high-end intensity (due to the delay time period). Accordingly, the bus capacitor CBUS will quickly discharge, such that the control circuit 64 becomes unpowered and the lamp tube 22 is extinguished, which, of course, is undesirable.
FIGS. 3A and 3B show example waveforms of the DC bus voltage VBUS and the lamp current ILAMP, respectively. As shown in FIG. 3A, the bus voltage VBUS is characterized by some low-frequency voltage ripple having a frequency approximately equal to twice the frequency of the AC power source 15 (e.g., approximately 120 Hz). The control circuit 64 is typically characterized by a corner frequency of approximately 10-20 Hz, and thus controls the inverter circuit 56 to adjust the operating frequency fOP of the square-wave voltage VSQ at a relatively slow rate in response to the lamp current control signal VILAMP. Since the corner frequency (i.e., approximately 10-20 Hz) is less than the frequency of the voltage ripple of the bus voltage VBUS (i.e., approximately 120 Hz), the operating frequency fOP of the square-wave voltage VSQ (and thus the lamp current ILAMP) is maintained relatively constant over short time intervals (e.g., during a single half-cycle THC of the AC power source 15, i.e., approximately 8.33 msec). As a result, the lamp current ILAMP has an envelope IENV that is characterized by the frequency of the voltage ripple of the bus voltage VBUS (as shown in FIG. 3B). This fluctuation (or ripple) in the envelope IENV of the lamp current ILAMP can cause undesirable flicker in the lamp tube 22.
Accordingly, there is a need for a dimmable screw-in compact fluorescent lamp having an integral electronic ballast circuit that avoids the disadvantages of the prior art circuits.